This invention relates to image data processing and more particularly to converting between image data of a plurality of differing formats. Specifically, the invention relates to the conversion between defined forms of standardized image data, wherein one form involves single-line coding and another form involves multiple-line coding in a manner optimized for speed of processing. The invention has particular application in facsimile data transmission and in bit-mapped image reproduction, such as in connection with electrostatic (laser) graphics printers.
Image data is an important data form for conveying information of both text and graphic content. The current market for laser printers alone is valued at more than one billion dollars. The facsimile market has a similar large market value.
Image data is typically coded as either a compressed code or a bit-mapped code. A single standard page-sized bit-mapped data image having a resolution of 400 lines per inch requires sixteen million bits of storage. Transmission of a bit-mapped image a bit at a time over conventional telephone-grade 3000 Hz bandwidth communication media is considered to be prohibitively time-consuming.
In order to reduce the size of image data, various compression techniques have been adopted. In the facsimile art, for example, image data is reduced to one of several types of compressed code form, such as Group III (Modified Huffman or MH) or Group IV (MMR) prior to transmission.
Frequently it is necessary to convert compressed image data from one compressed code form to another compressed code form in order to overcome the lack of compatibility among types of equipment. In the past it has been considered that conversion between Group III and Group IV coded images is so complex that it could not be done except by creation of a complete bit-mapped image version of the source data and applying conventional facsimile encoding process to obtain the image in the destination code format. Nevertheless, a straight-forward manipulation of a full-page bit-mapped image at 400 lines per inch requires processing of 16 millions bits of data. Such a process is extremely time-consuming and cumbersome. A bit-mapped data image is typically not further manipulated, except that it may be used to reproduce an image output on an electrostatic printer or the like. Thus, conventional facsimile and printing technology has encountered a barrier in the trade-off between image resolution and the speed of image processing. Whereas standard facsimile image data is considered too complex for any sort of meaningful image data manipulation, bit-mapped image data is considered to be simply too massive to be manipulated efficiently. Nevertheless, standardized facsimile image data is attractive because facsimile is becoming universally acceptable as a mode for transferring information. Thus, needs exist to provide better techniques for converting facsimile coded compressed image data among various formats, to speed the process of image reproduction and to process the image information in general. Further unexpected benefits might also accrue with the solution to these problems.